Airbridges of various shapes have been known, such as those disclosed, e.g., in Japanese Laid-Open Patent Publication Nos. H10-65006, H08-107120, 2008-270617, S61-194851, H08-288384, H08-8340, and H03-105949. An airbridge is typically a three-dimensional wiring structure formed on a substrate so as to use air between the structure and the substrate as an insulator. More specifically, it is a bridge-shaped wiring structure of conductive material formed on a substrate so as to straddle various components on the substrate, such as wiring conductors and semiconductor elements.
In the formation of an airbridge extending over components, the first step is to apply a layer of spacer material (e.g., a resist) over these components, as described, e.g., in the above Japanese Laid-Open Patent Publication No. S61-194851. The next step is to form a layer of airbridge-forming conductive material on this resist layer, followed by the step of removing the resist layer. Thus, the resist layer serves as a spacer for forming a space under the airbridge. As such, the shape and size of the resist layer determine the width of the airbridge, the distance spanned by the airbridge (or the length of the airbridge), and the height of the airbridge.
A typical cross-sectional shape of an airbridge is a rectangle (more specifically, a rectangular U-shape with its open ends facing the substrate), as is known in the art.
Further, for example, various U-shaped cross-sectional shapes of airbridges which include a curve have been proposed (i.e., these airbridges have a curved surface), as in the above Japanese Laid-Open Patent Publication Nos. H10-65006 and 2008-270617.
One advantage of the airbridge structure is that it protects from moisture the components (e.g., semiconductor active devices such as transistors) disposed below and covered by the airbridge. The area of the substrate surface located below and covered by the airbridge (which area may be referred to as the “coverage area”) can be increased by increasing the width, or depth, of the airbridge (which herein refers to the dimension that is perpendicular to the height and the direction of the span of the airbridge). The greater the area of the substrate surface covered by the airbridge, the more protected the covered semiconductor active devices are from moisture.
However, simply increasing the width of a conventional rectangular airbridge (which requires a corresponding increase in the width of the airbridge-forming material layer) can cause a problem in its manufacturing process. Specifically, in the airbridge manufacturing process, a layer of airbridge-forming material is formed on a resist serving as a spacer, and then this resist is removed to produce the rectangular airbridge. However, resist material is likely to be left in the inner corners of the rectangular airbridge if the airbridge has an increased width (as compared to conventional constructions). The reason for this is that the resist must be formed along the increased width of the airbridge, making it difficult to remove the resist from between the airbridge and the substrate. One method for preventing such spacer material from being left in the corners of a rectangular airbridge is to increase the area of the opening under the airbridge (i.e., the area of the opening formed between the airbridge and the substrate). However, simply increasing the size of the rectangular airbridge to increase the area of this opening might result in a decrease in the strength of the airbridge.
Another method is to form an airbridge having curved surfaces, as is known in the art and described above. This prevents spacer material from being left in the inner corners of the airbridge, as well as avoiding a decrease in the strength of the airbridge. In practice, however, the formation of such airbridges requires a heating process to thermally deform the resist so that it has curved surfaces, as disclosed in the above Japanese Laid-Open Patent Publication No. H10-65006. This heat treatment adversely affects components around the airbridge. Specifically, heat from the heating process may cause a change in the transistor characteristics after they have been optimized in the transistor manufacturing process.
Thus it has been difficult, by conventional techniques, to manufacture an airbridge having a wide coverage area while conforming to various requirements as to the strength and other characteristics of the airbridge structure and as to the manufacturing process.